The immune system, with its complex network of antigen-driven and non-antigen-driven pathways is quite dynamic with regards to the nature and type of elicited responses. The final products of the numerous available mechanisms can function either synergistically or antagonistically with respect to one another. In response to stimulation with tumor antigens, either effector (rejection) or suppressor (progression) responses can eventually dominate. Thus, for many reasons, it is necessary to consider characteristics of tumors, the immunological status of the host and environmental influences (both to the host and the tumor) concomitantly in any valid and complete study. In this context, our laboratory investigates questions related to the immunological balance which exists between a host and its tumor. These include a determination as to whether Langerhans cells are involved in the induction of tumor-specific suppressor T cells by epidermal ultraviolet light exposures. In addition, we are currently evaluating the interrelationship which exists between the MHC and ultraviolet light-mediated immunoregulatory influences as well as the heterogeneity of immunological regulatory mechanisms which influence tumor progression. Tumor cell cloning is proving to be a valuable tool to study tumor composition. Clones (the product of single tumor cells) are proving to be homogeneous with regards to tumor antigen expression and heterogeneous with regards to tumorigenic properties when compared with the parental tumor.